ROOM-TEMPERATURE ULTRAVIOLET-LASER EMISSION FROM ZNO NANOCRYSTAL THIN-FILMS GROWN BY LASER MBE

We have observed ultraviolet laser emission from ZnO nanocrystal thin films at room temperature. ZnO films were epitaxially grown on sapphir e substrates by laser molecular beam epitaxy. Closely packed and hexag onally shaped nanocrystals were formed in a spiral growth mode presuma bly due to a large lattice mismatch (similar to 18% between ZnO and sa pphire. Photoluminescence of these films clearly showed a sharp emissi on due to free excitons at room temperature. Above a threshold intensi ty as small as 24 kW cm(-2) of pumping laser pulses (355 nm, 15 ps), w e observed stimulated emission at 3.2 eV, increasing with a power of e ight as increasing pumping intensity. This emission is shown to be due to an exciton-exciton collision process. Excitons are confined in the se nanocrystals to show the giant oscillator strength effect and resul ting in an excitonic stimulated emission even at room temperature. (C) 1998 Elsevier Science S.A. All rights reserved.